The merozoite and the infected erythrocyte interact in a receptor- specific manner with erythrocyte and endothelium, respectively, to promote invasion of erythrocytes and cytoadherence to endothelium. These steps are critical in the pathogenesis of malaria and are targets for vaccines and drugs to block invasion and cytoadherence. We are identifying the P. knowlesi, P. vivax and P. falciparum parasite ligands and the host receptors for invasion and the molecular basis of antigenic variation. An antigen on the erythrocyte surface involved in antigenic variation and cytoadherence has been the focus of study because of its importance to pathogenesis, including its having the same motif as the parasite molecule involved in erythrocyte invasion. The components in the junction and the signaling after malaria merozoites and Toxoplasma gondii make contact with their host cells is also under study.